Electromagnetic switch having a flexible permanent magnet armature

ABSTRACT

An electromagnetic switch provided with at least one magnetic circuit comprising a permanent magnet, the polarity of which is reversible by means of a magnetizing coil. The permanent magnet is provided with pole pieces which magnetically cooperate with a flat flexible magnet of a movable armature. The flexible magnet may be provided with soft-iron insertion pieces in the regions of the poles. Preferably, the coercive force Hc of the main magnet is smaller than 1,500 oersted and that of the armature magnet is greater than 2,000 oersted.

0 l I I I I States Patent [1 1 [111 3,715,695 Hofman 1 Feb. 6, 1973 [54] ELECTROMAGNETIC SWITCH [56] References Cited HAVING A FLEXIBLE PERMANENT UNlTED STATES PATENTS MAGNET ARMATURE 3,089,064 5/1963 De Bennetot .335 230 ux [75] Inventor: Wytze Hofman, Emmasingel, Eind- 3,218,523 11/1965 Benson hoven, Netherlands 3,585,547 6/197] Sturman ..335/230 X [73] Assignee: U.S. Philips Corporation, New Primary Examiner-George Harris York, NY. Att0rney-Frank R. Trifari PP NO-I 174,731 An electromagnetic switch provided with at least one magnetic circuit comprising a permanent magnet, the polarity of which is reversible by means of a magnetiz- [30] Foreign Apphcatmn Prwmy Data ing coil. The permanent magnet is provided with pole Aug. 30, 1970 Netherlands ..7012890 pieces which magnetically cooperate with a flat flexible magnet of a movable armature. The flexible mag- 52 U.S. Cl. ..335 229, 335/279, 335/303 net may be Provided with Soft-iron insertion Pieces in [51] Int. Cl ..H0lf 7/08 the regions of the Poles Preferably the coercive force [58] H of the main magnet is smaller than 1,500 oersted Field of Search ..335/229, 230, 234, 279, 303

and that of the armature magnet is greater than 2,000 oersted.

7 Claims, 6 Drawing Figures VW/f /X/// V/Z/ A /V/ A v// Fig.4 31

' INVENTOR. wnzs HOFMAN AGENT ELECTROMAGNETIC SWITCH HAVING A FLEXIBLE PERMANENT MAGNET ARMATURE The invention relates toan electromagnetic switch including at least one magnetic circuit which comprises a permanent magnet provided with pole pieces which define an active air-gap. Part of the magnetic circuit is surrounded by a magnetizing coil, and a movable armature which is disposed opposite the air-gap and magnetically cooperates with the magnetic circuit.

Such a switch is described in German Pat. specification No. 1,194,980. The armature of this switch consists of a single soft-iron strip which is attracted when the magnetizing coil is energized. Closure of the armature is effected with a certain acceleration and with a shock. I

The present invention provides a flat permanent magnet armature which is made of finely divided permanent-magnetic material bonded in a resilient synthetic material. This provides the advantage that in closing the armature the shock is absorbed by the resilience of the armature magnet.

Preferably the armature magnet has its rear surface secured to a soft-iron strip which serves as a short circuit for the lines of magnetic force which'emerge from the rear surface of the armature magnet. v

In one embodiment of the present invention when the switch is in the closedposition both pole pieces contact the armature magnet, each pole piece magnetically co-operating with a magnetic pole of opposite polarity.

In another embodiment, in the closed positionof the switch only one pole piece contacts the armature magnet, the other pole piece magnetically co-operating with the soft-iron strip of the armature.

In order to reduce the influence of the stray field produced in the air-gap an auxiliary magnet is arranged in this air-gap between the pole pieces.

In a switch according to the invention which provides particularly satisfactory operation each part of the armature magnet which co-operates with the pole pieces is provided with a flat soft-iron insertion piece which serves as a pole piece.

When the coercive force of lie of the main magnet is smaller than 1,500 oersted and that of the armature magnet is greater than 2,000 oersted, at a sufficiently large current pulse through the magnetizing coil the main magnet will have its polarity reversed. Consequently, magnetic poles of opposite sign also may become located opposite the armature magnet, giving rise to a large force of repulsion with a consequent increase in the switching rapidity.

Embodiments of a switch according to the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings,'in which FIG. 1 is a cross-sectional view of an embodiment of the invention in which the armature magnet cooperates with either pole piece,

FIG. 2 is a cross-sectional view of another embodiment of the invention in which the armature magnet magnetically co-operates with one pole piece,

FIG. 3 is a cross-sectional view of a modification of the embodiment shown in FIG. 2 in which the armature magnet is provided with an insertion piece, I

FIG. 4 is a cross-sectional view of a modification of the switch of FIG. 2 provided with an auxiliary magnet,

FIG. 5 is a cross-sectional view of a switch according to the invention in which the armature magnet is provided with a different insertion piece, and

FIG. 6 shows a tape drive for a magnetic recording and play-back apparatus provided with a switch according to the invention.

Referring now to FIG. 1, a switch comprises a magnetic circuit 1 and an armature 2 which co-operate magnetically. The magnetic circuit includes a cylindrical permanent main magnet 3, which at either end is provided with a soft-iron pole piece 4 and 5 respectively. The main magnet 3 is axially magnetized so that the pole piece 5 is a north pole and the pole piece 4 is a south pole. The. polepieces are L-shaped, coplanar limbs 6 and 7 defining an air-gap 8. The cylindrical main magnet 3 is surrounded substantially throughout its length by a magnetizing coil 9.

The armature comprises a flat soft-iron strip 11 arranged for rotation (not shown).

To the strip there is secured a flat auxiliary magnet 12 which in the closed position of the switch is in contact with the limbs 6 and 7 of the pole pieces 4 and 5 respectively substantially throughout its entire upper surface. This upper surface comprises two unlike magnetic poles which each are disposed opposite one of the limbs 6 and 7 and have signs opposite to those of the magnetic poles of the contacting limbs; thus a north pole N lies opposite the south pole S of the limb 6, and a south pole S lies opposite the north pole N of the limb The main magnet 3 is made of the material known under the trade name Ticonal 700 having a coercive force H 700 oersted and a (BH),,,,,,== 7.10

In the case ofa main magnet having a length of 6 mm and a diameter of 5 mm a current pulse of 40 amperes through the magnetizing coil which consisted of 60 turns proved to be sufficient to reverse the polarity of the main magnet.

The armature magnet 12 is made of a plasto-ferrite known under the trade-name P 120, thepolarity of which is not readily reversible. The coercive force He is 2,200 oersted and the (BI-I),,,,, is 12' 10.

Under normal conditions, i.e., when no current flows through the magnetizing coil 9, the switch is closed. The magnetic poles of the magnetic circuit 1 constituted by the limbs 7 and 6 of the pole pieces 5 and 4 attract the magnetic pole of opposite sign of the armature magnet 12. The path of the lines of magnetic force is indicated by arrows in FIG. 1. The magnetic circuit is closed by a soft-iron armature 11.

When a sufficiently large current pulse is applied to the magnetizing coil 9, the polarity of the main magnet 3 is reversed, i.e., the signs of the magnetic poles formed by the limbs 6 and 7 are reversed. These magnetic poles and the contacting poles of the armature magnet now are of like sign and hence they repel one another. Thus the switch is opened. A subsequent current pulse through the magnetizing coil 9 again causes the polarity of the main magnet 3 to be reversed, and

the switch is closed. The force of this closure is absorbed by the resilience of the material of the armature magnet.

FIG. 2 shows a modified embodiment of the switch according to the invention.

The magnetic circuit 1 comprises a main magnet 3 having 'pole pieces 4 and 5-which is surrounded by a magnetizing coil. The armature comprises, similarly to that of the preceding embodiment, a soft-iron strip ll and an armaturemagnet 'l6. The pole piece 4 is L- shaped, however, the limb 14 which contacts'the armature magnet 16 is lengthened. Its length is nearly equal to the length of the armature magnet 16. The pole piece consists of a single flat limb. The armature magnet 16 is magnetized in the transverse direction.

The surface of the armature. magnet 'which cooperated with the limb 14 is provided with a north pole N. I I 1 In the closed position of the switch the limb 14 contacts the flat magnet 16. An end-face of the pole piece 5 is in contact with part of the flat armatui'e strip 11. This part forms a south pole which magnetically co-operates with the rriagne tidpolie N of thele'iid face l5ofthepolepiece5 I I FIG. 3 shows a similar construction, howev'en 'the armature magnet is of a different structure. Thearmature magnet 20 is lengthened and has an extension 22 of reduced height at the region of the pole piece 5, the end 15 of which is shortened. The height of the extension 22 is made up to the height of the remainder of the armature magnet by a soft-iron insertion piece 23. The extension 22 is also magnetized in the transverse direction, however, its magnetization is opposite to that of the part 20.

In order to greatly reduce the stray field, in the switch shown in FIG. 4, which is largely similar to that shown in FIG. 2, an auxiliary magnet'30 is interposed between an end 31 of the limb 14 and a lateral face 32 of the pole piece 5. This auxiliary magnet 30 is flat'and magnetized in the transverse direction. It is made of plactoferrite P 120 and has a north pole N facing the lateral face 32 and a south pole S facing the end 31. The auxiliary magnet compels the lines of magnetic force to follow more accurately the path shown by ar- TOWS The switch shown in FIG. 5 is of a different structure. However, its operation is equal to that of the preceding embodiments.

The magnetic circuit comprises a main magnet 40 arranged centrally ofa soft-iron circuit comprising an upper plate 41, a lower plate 42 and a pot-shaped rim 43. The main magnet 40 is surrounded by a magnetizing coil 44. The-upper plate 41 together with the rim 43 defines an annular air-gap 45.

The armature 2 which is shown in cross-section comprises a soft-iron strip 46 and an armature magnet 47, the edge portion 48 of whichisthinner thanthe in a direction opposite to that of he central portion 49.

A soft-iron annular insertion piece 50 is arranged on the edge portion of the armature magnet. The upper plate 41 magnetically'co-operates with the central portion 49,...an'd a rim;43co-operates with the insertion piece 50.

FIG. 6 shows a use of the switch according to the invention. The switch operates the drive of a magnetic recording and/or play-back apparatus.

The switch 60 comprises a magnetic circuit 61 of the aforedescribed type which is fixed. The armature 62 is arranged for rotation about a spindle 63. To its limb 64 15 secured a rubber thrust roller 65 which in the closed position of the switch presses a magnetic tape against a "capstan, 66. The capstan 66 is arranged centrally in a What is claimed is:

' wherein the armature magnet at its rear surface is secured to a soft-iron strip.

3. An electromagnetic switch as claimed in claim 1 wherein both pole pieces are in contact with the armature magnet when the switch is in the closed condition, each pole piece cooperating with a magnetic pole of opposite polarity.

4. An electromagnetic switch as claimed in claim 2, wherein only one pole piece is in contact with the armature magnet when the switch is in the closed position, the other pole piece magnetically cooperating with the soft-iron strip of the armature.

5. An electromagnetic switch as claimed in claim 4,

furthercomprising an auxiliary magnet arranged in the air-gap between the pole pieces.

W 6. An electromagnetic switch as claimed in claim 1 further comprising a flat soft-iron insertion piece which serves as a pole piece provided on each part of the armature magnet which magnetically cooperates with the pole piece.

' 7. An electromagnetic switch as claimed in claim 1 wherein the coercive force I-I of the main magnet is smaller than 1,500 oersted and the coercive force of the armature magnet is greater than 2,000 oersted.

Patent No. 3715695 7 mg UNITED STATES PATENT omen CETWMATE 9F ECH@N lnventorm WIJTZE HOF'MAN It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading, change Priority Data from "Aug. 30, 1970" to Aug. 31, 1970- Signed end sealed this 3rd day of July 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Rene 'Fegtmeyer Attesti ng Officer Acting Commissioner of Patents mg UNITED STATES PATENT @FFECE v CERTIWQATE OF CURREUNUN Patent NO- 3715695 a Dated r a w v Inventorw WIJTZE HDF'MAN It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading, change Priority Data from "Aug. 30, 1970" to --Aug. 31, 1970- Signe'd and sealed this 3rd day of July 19.75.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Rene 'Fegtmeyer Attesti g Officer Acting Commissioner of Patents 

1. An electromagnetic switch provided with at least one magnetic circuit which comprises a permanent magnet provided with pole pieces which define an active air-gap, part of the magnetic circuit being surrounded by a magnetizing coil, and a movable armature which is disposed opposite the air-gap and magnetically cooperates with the magnetic circuit, said armature comprising a flat permanent magnet made of finely divided permanent magnet material bonded in a resilient synthetic material.
 1. An electromagnetic switch provided with at least one magnetic circuit which comprises a permanent magnet provided with pole pieces which define an active air-gap, part of the magnetic circuit being surrounded by a magnetizing coil, and a movable armature which is disposed opposite the air-gap and magnetically cooperates with the magnetic circuit, said armature comprising a flat permanent magnet made of finely divided permanent magnet material bonded in a resilient synthetic material.
 2. An electromagnetic switch as claimed in claim 1, wherein the armature magnet at its rear surface is secured to a soft-iron strip.
 3. An electromagnetic switch as claimed in claim 1 wherein both pole pieces are in contact with the armature magnet when the switch is in the closed condition, each pole piece cooperating with a magnetic pole of opposite polarity.
 4. An electromagnetic switch as claimed in claim 2, wherein only one pole piece is in contact with the armature magnet when the switch is in the closed position, the other pole piece magnetically cooperating with the soft-iron strip of the armature.
 5. An electromagnetic switch as claimed in claim 4, further comprising an auxiliary magnet arranged in the air-gap between the pole pieces.
 6. An electromagnetic switch as claimed in claim 1 further comprising a flat soft-iron insertion piece which serves as a pole piece provided on each part of the armature magnet which magnetically cooperates with the pole piece. 